Quick answer: capacity for a horizontal cylindrical tank is π × (diameter ÷ 2)² × length. But to read a dipstick you need the circular segment formula, because a quarter of the depth is only about 19.6% of the fuel. Enter your reading above and the calculator handles it.
Dipstick depth to fuel remaining
This is the table to tape inside the boiler room door. It applies to any horizontal cylindrical tank, whatever its size:
| Dipstick reading (% of tank height) | Fuel remaining | In a 275 gal tank | In a 500 gal tank |
|---|---|---|---|
| 10% | 5.2% | 14 gal | 26 gal |
| 20% | 14.2% | 39 gal | 71 gal |
| 25% | 19.6% | 54 gal | 98 gal |
| 30% | 25.2% | 69 gal | 126 gal |
| 40% | 37.4% | 103 gal | 187 gal |
| 50% | 50.0% | 138 gal | 250 gal |
| 60% | 62.6% | 172 gal | 313 gal |
| 75% | 80.4% | 221 gal | 402 gal |
| 90% | 94.8% | 261 gal | 474 gal |
Look at the 25% row and then the 75% row. The bottom quarter of the tank's height holds 54 gallons; the top quarter holds 54 as well (275 − 221). But the two middle quarters hold 167 between them. Fuel disappears slowly at first and then alarmingly fast once the level drops below the midline — which is exactly the wrong way round for anyone judging by eye.
Common fuel tank sizes
| Tank | Shape | Dimensions | Usable |
|---|---|---|---|
| 275 gal heating oil | Vertical oval | 27 wide × 44 tall, 60 in long | ≈ 240 gal |
| 330 gal heating oil | Vertical oval | 27 wide × 44 tall, 72 in long | ≈ 290 gal |
| 500 gal propane | Horizontal capsule | 37 in ⌀, 119 in long | 400 gal (80%) |
| 1,000 gal propane | Horizontal capsule | 41 in ⌀, 193 in long | 800 gal (80%) |
| Farm diesel tank | Horizontal cylinder | 4 ft ⌀, 10 ft long | ≈ 900 gal |
| Transfer tank (pickup) | Rectangular | 48 × 20 × 19 in | ≈ 75 gal |
Usable is always less than geometric capacity. Heating oil deliveries stop at roughly 90% to leave expansion room, the suction line sits an inch or two above the floor so sludge is not drawn up, and propane is limited to 80% by regulation because liquid propane expands sharply as it warms.
Planning a delivery
The practical question is almost always "how long have I got?" Work it in three steps:
- Read the stick and convert. Say 14 inches in a 44-inch oval 275-gallon tank — about 78 gallons.
- Estimate the burn rate. A typical home in cold weather uses 4 to 7 gallons a day; check by dividing a past delivery by the days it lasted.
- Divide, then subtract a buffer. 78 ÷ 5.5 ≈ 14 days — but order at a week, because suppliers get busy in cold snaps and running a tank dry pulls sludge into the line and usually means a service call to re-prime.
For diesel stored on a farm or site, the same arithmetic applies with an extra consideration: diesel degrades over months, so sizing storage far beyond your turnover creates a fuel-quality problem rather than a supply cushion.
Common mistakes & pro tips
- Reading depth as a percentage of volume. The central error with any horizontal tank, and it always flatters the low end.
- Confusing a cylinder with a capsule. Propane tanks have domed ends that hold roughly a fifth of the capacity. Use the tank calculator's capsule mode for those.
- Assuming the tank is level. A tank sitting even slightly off level shifts the reading, especially on a long tank. Take the reading at the same point every time.
- Forgetting the unusable heel. The fuel below the suction line never comes out.
- Pro tip — calibrate the stick once. Run this calculator at 2-inch increments and mark the gallons directly on the dipstick. Every future reading takes two seconds.
- Pro tip — temperature. Heating oil expands about 0.05% per °F. A delivery on a cold day measures slightly less volume once it warms in the tank — a small effect, but real on large deliveries.
Frequently asked questions
How do I tell how much heating oil is left in my tank?
Measure the oil depth with a dipstick, then convert using the tank's shape — not by proportion. A standard 275-gallon oval tank is 44 inches tall, so a 12-inch reading is 27% of the height but only about 24% of the volume, roughly 64 gallons. Enter your reading in the calculator above for the exact figure.
Why is my fuel gauge not proportional to the depth?
Because most fuel tanks lie on their side. A horizontal cylinder is widest through the middle and narrow top and bottom, so the volume per inch of depth changes constantly. At a quarter of the tank's diameter you have about 19.6% of capacity, not 25% — and the last quarter of depth holds only about a fifth of the fuel.
How many gallons is a 275 gallon oil tank?
About 275 gallons brim-full — typically 27 in wide, 44 in tall and 60 in long with an oval cross-section. Usable oil is less: deliveries stop around 90% and the outlet sits above the tank floor, so plan on roughly 240 usable gallons.
Why are propane tanks only filled to 80 percent?
Liquid propane expands about 1.5% for every 10 °F of temperature rise. The 20% vapour space stops a tank filled on a cold morning from becoming liquid-full and over-pressurised later in the day. A 500-gallon propane tank therefore takes about 400 gallons.
When should I reorder heating oil?
Most suppliers suggest ordering at about a quarter tank. On a 275-gallon oval that is roughly 60 to 70 gallons, which at a cold-weather burn rate of 5 gallons a day is under two weeks of heat. Because depth is not proportional to volume, a tank that looks a quarter full by the stick is often emptier than it appears.